[Studies of the target cell problem in N-nitroso-N-methylurea induced leukemogenesis]

After i.p. application of 14C-nitrosomethylurea (NMU), mice were killed at different periods and the 14C-activity in various organs was determined by scintillation counting and by autoradiography. Contrary to expectations, bone marrow showed a significantly higher activity than the thymus, which is the supposed target-organ for the lymphatic leukemogenesis. The specificity was secured by examinations of the proliferation kinetics. The radioautographic results of bone marrow favorize the lymphatic cells as the target for NMU. The target-cell problem is discussed in respect to thymectomy examinations and recent results on nude mice. With high probability the thymus is not essential for lymphatic leukemogenesis.